Described below is a process for regulating the joule value of offgases from pig iron production plants having integrated CO2 removal plants, wherein at least one part of the offgas is discharged from the pig iron production plant as export gas, if necessary collected in an export gas container and subsequently thermally utilized in a gas turbine, wherein the offgas from the gas turbine is supplied to a waste heat boiler for the generation of steam. The process can equally be utilized for regulating the joule value of synthesis gas from plants for synthesis gas production having integrated CO2 removal plants, wherein at least one part of the synthesis gas is discharged from the plant for synthesis gas production as export gas, not, however, collected in an export gas container, but subsequently thermally utilized in a gas turbine, wherein the offgas from the gas turbine is supplied to a waste heat boiler for the generation of steam. Also described below is a plant for carrying out the process.
There are fundamentally two known common methods for the production of pig iron, which should also include the production of pig iron products: the blast furnace process and the smelting reduction process.
During the blast furnace process, firstly pig iron is produced from iron ore by coke. Furthermore, iron scrap can additionally be used. Then steel is produced from pig iron through further processes. The iron ore is mixed as lump ore, pellets or sinter together with the reducing agent (mostly coke, or also coal, e.g. in the form of a fine coal injection plant) and further components (limestone, slag formers, etc.) with the so-called burdens and subsequently charged into the blast furnace. The blast furnace is a metallurgical reactor, in which the batch column reacts in the counter flow with hot air, the so-called hot blast. By burning the carbon from the coke, the heat and carbon monoxide or hydrogen necessary for the reaction are produced, the hydrogen representing a significant part of the reduction gas and flowing through the batch column and reducing the iron ore. As a result, pig iron and slag are produced, which are periodically tapped off.
In the so-called oxygen blast furnace, which is also identified as a blast furnace having top gas or furnace gas recirculation, oxygenated gas with a proportion of oxygen (O2) of more than 90% by volume is blown into the blast furnace, during the gasification of coke or coal.
A gas purification (e.g. dust separator and/or cyclones combined with wet scrubbers, bag filter units or hot gas filters) must be provided for the gas emitted from the blast furnace, the so-called top gas or furnace gas. Furthermore, most of the time in the oxygen blast furnace, a compressor, which may have an after-cooler, is provided for the top gas, which is recirculated in the blast furnace, as well as a device for removing CO2, mostly by pressure swing adsorption, as known in the related art.
Further options for the embodiment of a blast furnace process are a heater for the reduction gas and/or a combustion chamber for the partial combustion with oxygen.
The disadvantages of the blast furnace are the demands on the input materials and the high emissions of carbon dioxide. The iron source and the coke which is used must be hard and in lumps, such that enough cavities remain in the batch column, which guarantee that the wind, which is blown in, flows through. The CO2 emissions represent a strong environmental burden. Therefore there are efforts to remove the blast furnace route. To be noted here are the sponge iron production based on natural gas (MIDREZ, HYL, FINMET®) as well as the smelting reduction processes (COREX® and FINEX® processes).
A smelter gasifier is used during the smelting reduction process, in which hot liquid metal is produced, as well as at least one reduction reactor, in which the source of the iron ore (lump ore, fine ore, pellets, sinter) is reduced with reduction gas, wherein the reduction gas is generated in the smelter gasifier by gasification of coal (and, if necessary, of a small proportion of coke) with oxygen (90% or more).
As a rule, during the smelting reduction process                gas purification plants (on the one hand for the top gas from the reduction reactor, on the other hand for the reduction gas from the smelter gasifier),        a compressor, which may have an after-cooler, for the reduction gas, which is recirculated in the reduction reactor,        a device for removing CO2, mostly by pressure swing adsorption, as known in the related art        as well as, optionally, a heater for the reduction gas and/or a combustion chamber for the partial combustion with oxygen are also provided.        
The COREX® process is a two-step smelting reduction process. The smelting reduction process combines the process of the direct reduction (pre-reduction of iron to sponge iron) with a smelting process (main reduction).
The equally well-known FINEX® process corresponds significantly to the COREX® process, however iron ore is introduced as fine ore.
The process is not only able to be used in pig iron generation, but also in synthesis gas plants. Synthesis gases are all gaseous mixtures containing hydrogen and mostly also containing CO, which should be used in a synthesis reaction. Synthesis gases can also be produced from solid, liquid or gaseous substances. In particular these include the coal gasification (coal is transformed with water vapor and/or oxygen to hydrogen and CO) and the production of synthesis gas from natural gas (transformation of methane with hydrogen and/or oxygen to hydrogen and CO). Beneficially, in the case of the coal gasification, the export gas storage, as is provided according to pig iron production plants, can be omitted, because the high synthesis gas pressure from the gasifier (mostly >20 barg, such as approximately 40 barg) can also equally be used in the gas turbine, where, as a rule, a gas pressure of approximately 20-25 barg is needed. The tail gas, which is rich in CO2, from the CO2 removal plant must, however, be compressed to the pressure of the synthesis gas flow by a compressor.
If the CO2 emissions into the atmosphere are to be reduced in the production of pig iron or in the generation of synthesis gas, these must be removed from the offgases from the pig iron or synthesis gas production and captured in a combined form (CO2 capture and sequestration (CCS)).
Until now the pressure swing adsorption (PSA), in particular also the vacuum pressure swing adsorption (VPSA), has principally been used to remove CO2. The pressure swing adsorption is a physical process for the selective deconstruction of gaseous mixtures under pressure. Special porous materials (e.g. zeolite, activated carbon, activated silicon oxide (SiO2), activated aluminum oxide (Al2O3) or the combined use of these materials) are used as a molecular sieve, in order to adsorb molecules according to their adsorption strengths and/or their kinetic diameter. During PSA, the fact that gases adsorb at various strengths to the surface is used. The gaseous mixture is introduced into a column under an exactly defined pressure. Now the undesirable components (here CO2 and H2O) and the recyclable material (here CO, H2 CH4) flow through the column, to a great extent unobstructed. As soon as the adsorbent is completely loaded, the pressure is reduced and the column is backwashed. An electric current for the preceding compression of the gas, which is recirculated and is rich in CO2, is needed to operate a (V)PSA plant.
The product gas flow after the pressure swing adsorption, which contains the recyclable material, still contains, for example, 2-6% by volume CO2 in the offgases from the pig iron generation. The tail gas flow from the (V)PSA plant still, however, contains relatively high reducing gas proportions (for example CO, H2), which are lost during the pig iron production.
The tail gas flow after the pressure swing adsorption, which contains the undesired components, is typically composed as follows in the offgases from the pig iron production:
Combination% by vol during VPSA% by vol during PSAH22.25.5N21.52.4CO10.916.8CO282.172.2CH40.70.9H2O2.62.2
The tail gas cannot simply be thermally utilized, because for that—due to the low and/or fluctuating joule values of, for example, ±50%—it would have to be augmented with other fuels. It can, for example, be added in its entirety to the so-called export gas, which is the part of the process gas, which is removed from the process of the pig iron or synthesis gas generation and is used for other purposes, for example as a fuel in a combined gas and steam power station, which is also identified as a combined cycle power plant (CCPP). Components of the export gas in the pig iron generation can be:                top gas and/or generator gas from a blast furnace, a reduction reactor (fluidized bed reactor) or a reduction shaft (fixed bed reactor)        so-called offgas from a reduction reactor (fluidized bed reactor)        so-called excess gas from a smelter gasifier        
The addition of tail gas from the CO2 removal to the export gas is, then, only beneficial if the joule value of the export gas is so high that it does not drop under a value after the addition of the tail gas that is too low for the subsequent use of the export gas.
A reduced joule value of the export gas subsequently decreases the efficiency of a power station supplied with the export gas, for example in a combined cycle power plant, because of the high compression of gaseous fuel and because of the lower efficiency of the gas turbine. In a steam power station or furnace the flame temperature would be reduced during the combustion.
If an addition of the tail gas from the CO2 removal to the export gas is not beneficial, this was until now combusted in its entirety on a hot flare. This does not only have the disadvantage that heat, which is produced during flaring, is lost, but also that considerable gas emissions in the form of carbon monoxide CO, hydrogen sulphide H2S, etc., can be produced by incomplete combustion of the tail gas in the hot flare.
Another problem in using export gas from plants for the production of pig iron and synthesis gas is that the joule value of the export gas fluctuates. Therefore the export gas is captured in an export gas container having a large volume, e.g. in the size of 100,000 m3, before being supplied to a consumer, such as a power station, in order to homogenize the gas composition. In order to achieve a constant joule value having a fluctuation margin of +/−1-2%, until now waste nitrogen from an air deconstruction plant was added when the joule value deviated upwards from the desired constant value. Coke oven gas (for example from the pyrolysis of hard coal to coke for the blast furnace) was added when the joule value deviated downwards.
A corresponding process for balancing the joule value is shown in AT 507 525 B1. According to this publication the export gas is supplied to a buffer unit, where the regulation of the joule value occurs, according to which the joule value is raised by the addition of smelter gas or natural gas and reduced by the addition of nitrogen or water vapor.
In AT 507 525 B1, the accumulating tail gas from a CO2 removal unit is collected in a particular storage unit, wherein the joule value in the captured tail gas is balanced. The tail gas, which is captured in advance, is supplied to a waste heat boiler, where steam is generated through the combustion of the tail gas, the steam driving a steam turbine and a generator. A part of the export gas can be supplied to the tail gas in the storage unit, the export gas having passed through a top gas pressure recovery turbine.
When carrying out the combustion of the tail gas in the hot flare according to AT 507 525 B1, it is avoided, although it is disadvantageous, that high value gaseous fuel, such as smelter gas, which can be disposed of in the plant or specially provided natural gas, which is not available in the plant, is used for the regulation of the joule value of the export gas.
Therefore described below is a process to regulate the joule value of the export gas, which manages with a small addition of high value gaseous fuel.